(1) Field of the Invention
This invention relates to a mask, and more particularly relates to a mask applied in sequential lateral solidification (SLS) processes.
(2) Description of the Prior Art
A liquid crystal display (LCD) with the advantages of slim size, low power consumption and radiation damage, has become a preferred choice among various displaying products such as CRT displays. For the same reason, the LCD has been widely used in various electronic devices such as desk top computers, personal digital assistants, note books, digital cameras, cell phones, and etc.
FIG. 1 shows a typical active matrix liquid crystal display (AMLCD) panel 10 with a plurality of pixel devices 12 arrayed in matrix. Each pixel device 12 has a connection with a thin film transistor (TFT) 14 operated as a switch for charging and discharging the pixel device 12. The source electrode of the TFT 14 is electrically connected with a source driver (not shown) through a signal line 16. The gate electrode of the TFT 14 is electrically connected to a scan driver (not shown) through a gate line 18. Displaying signals are transformed into source driving voltage (Vs) and gate driving voltage (Vg) applied to the source electrode and gate electrode of the TFT respectively for generating images.
Due to the temperature limitation of the glass substrate, in traditional manner, the TFTs 14 formed on the LCD panel 10 must be amorphous silicon TFTs. However, the switching speed, the electric characters, and the reliability of the amorphous silicon TFTs are not qualified as being used in the drivers of the LCD panel 10 for controlling the display. Instead, polysilicon TFTs are suggested to be used in the drivers to achieve a higher operation speed. In order to meet the dilemma, additional silicon chips for allocating the drivers must be used to connect to the LCD panel 10 through some pipelines.
There are two reasons that the polysilicon TFTs fabricated on the glass substrate are demanded in present. First, the pixel devices need a higher switching speed for larger LCD panels. Second, the drivers must be formed on the glass substrate for a slimmer LCD panel. Therefore, the need of forming high quality polysilicon layers on the glass substrate has become urgent.
FIG. 2 shows a traditional low temperature polysilicon (LTPS) fabrication process. First, an amorphous silicon layer 120 is formed atop a substrate 100. Then, laser illumination is applied on the amorphous silicon layer 120 to form a melted layer 122 near the top surface of the amorphous silicon layer 120. The amorphous silicon material right under the melted layer 122 is utilized as seeds 124 growing upward to create grains 126. Due to the limitation of the thickness of the melted layer 122, the grain size is less than 1 micron and the promotion of the electric ability for the resultant TFTs is limited.
In order to form larger grains, the lateral solidification process is developed as shown in FIG. 3. The lateral solidification process uses a mask 200 for melting a predetermined region A within the amorphous layer 120. A lateral temperature gradient is generated in the melted region A. Thus, the amorphous silicon material close to the edge of the melted region A is utilized as seeds growing toward the center of the melted region A to generate larger grains 128.
FIG. 4A shows a typical mask 300 utilized in the sequential lateral solidification (SLS) process. As shown, the mask 300 has a plurality of first transparent regions 310 lined in rows, and a plurality of second transparent regions 320 lined in rows. Each first transparent region 310 is aligned to the shielded region between two neighboring second transparent regions 320.
FIGS. 4B and 4C show some variety of the typical masks applied in the SLS process. Other than the transparent regions 310, 320 of the mask 300 of FIG. 4A, which characterized with the symmetrical V-shaped front edge and V-shaped rear edge, these masks 300a,300b have transparent regions thereon being defined by the front edge and the rear edge with symmetrical trapezoid-shaped or semicircle-shaped, respectively.
FIG. 5 depicts the SLS process using the mask 300 of FIG. 4A. In the first illumination step (as defined by the dash line), laser illumination melts the amorphous silicon layer through the first transparent regions 310 and the second transparent regions 320 of the mask 300 to form a plurality of first crystallized regions A1. FIG. 6A shows an enlarged view of the first crystallized region A1 formed in the amorphous layer in the present illumination step. Affected by optical interference and diffraction near the boundary of the transparent region 310,320, a boundary portion a with grains of random orientation and size exists in the first crystallized region A1, and only the central portion b of the first crystallized region A1 has unified lateral grains. Therefore, it is understood that the length L of the central portion b can not be greater than the length L1 of the lengthwise edge of the first crystallized region A1.
In the second laser illumination step defined by the solid line of FIG. 5, the mask 300 moves rightward to have the first transparent region 310 aligned to the amorphous layer between the first crystallized regions A1. It is noted that the width of the first transparent region 310 is greater than the interval between two neighboring second transparent regions 320. Thus, as shown in FIG. 6C, the second crystallized region A2 generated in the present illumination step partially overlaps the first crystallized regions A1. Thereby, the random portion a near the lengthwise edges of the first crystallized region A1 is re-crystallized to improve the uniformity of the grains therein.
As to the improvement of the random portion a near the front edge and the rear edge of the first crystallized region A1, a limitation of the movement of the mask between two sequential steps of the SLS process exists. In detail, as shown in FIG. 6C, the distance D between the first crystallized region A1 generated in the first laser illumination step and the third crystallized region A3 generated in the second illumination step must be smaller than the length L of the central portion b to make sure the overlap between the first crystallized region A1 and the third crystallized region A3 are sufficient to eliminate the random portion a near the front edge of the first crystallized region A1. The fabrication speed of the SLS process is limited thereby. Meanwhile, the increasing of the overlapping area also increases the probability of agglomeration to result holing in the amorphous layer.
Accordingly, a mask utilized in the SLS process is provided in the present invention not only for eliminating the random portions but also for increasing the fabrication speed.